The present invention relates generally to golf balls having certain relationships among the densities of the various layers therein. The relationships among the densities may cause the golf ball to have certain values of its moment of inertia, which may result in favorable play characteristics.
Conventional multi-piece solid golf balls commonly include a solid resilient core having single or multiple layers, and at least one cover layer molded on the solid core. The solid core for a multi-piece solid golf ball is often formed from a combination of materials such as polybutadiene and other rubbers crosslinked with zinc diacrylate or zinc dimethacrylate. The cover is typically made of ionomeric resins that impart toughness and cut resistance.
Ionomeric resins are generally ionic copolymers of an olefin, such as ethylene, and a metal salt of an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid or maleic acid. Metal ions, such as sodium or zinc, are used to neutralize some portion of the acidic group in the copolymer. Ionomeric resins often exhibit useful properties, such as durability, for golf ball cover construction.
However, while ionomeric resins may have favorable durability, they also may exhibit unfavorable playability. Specifically, ionomeric resins tend to be quite hard. Ionomeric resins thus may lack the degree of softness required to impart the spin necessary to control the ball in flight. Namely, ionomeric resin covers do not compress as much against the face of the club upon impact due to their high hardness, thereby producing less spin. In addition, the harder and more durable ionic resins lack the “feel” characteristic associated with softer covers, such as traditional balata covers.
As is generally known, one property of a golf ball that may affect its spin is its moment of inertia. Moment of inertia, also referred to as “MOI” in the art and herein, is a measure of the resistance to twisting about a central axis. The higher the MOI of an object, the more force will be required to change the object's rotationally velocity. Conversely, the lower the MOI, the less force will be needed to change how fast the object rotates.
A golf ball having a high moment of inertia may exhibit advantageous play characteristics. For example, such a golf ball will typically have a lower rate of spin upon initially being struck by a golf club than a golf ball having a lower moment of inertia, as the high moment of inertia will initially resist the increase in the golf ball's rate of spin. Lower initial spin may result in the shot having a greater total distance. At the same time, the golf ball having a high moment of inertia may also have an increased rate of spin later during the flight path of the shot as compared to a golf ball having a lower moment of inertia, as the rate of spin slows from its maximum at a lower rate. Increase spin at this stage of the shot may result in better control on the green, and may also reduce the undesirable effects of cross-winds on the golf ball's trajectory.
Golf balls with increased moment of inertia are known in the art. For example, U.S. Pat. No. 6,939,249 to Sullivan discloses a “Golf Ball Having a High Moment of Inertia,” the disclosure of which is herein incorporated by reference in its entirety. However, known golf balls are generally limited to the certain constructions and materials used to achieve the high moment of inertia.
Therefore, there exists a need in the art for a golf ball having an advantageous construction that leads to improved spin characteristics.